In recent years, in luminaires, more and more illumination light sources such as an incandescent lamp and a fluorescent lamp are replaced with energy-saving and long-life light sources such as a light-emitting diode (LED). Further, new illumination light sources such as an EL (Electro-Luminescence) and an organic light-emitting diode (OLED) are developed. Optical outputs of these illumination light sources depend on a current value. Therefore, if lighting is turned on, a power-supply circuit that supplies a constant current is necessary. If the lighting is dimmed, an electric current to be supplied is controlled.
A two-wire dimmer is configured to control a phase in which a triac is turned on. The two-wire dimmer is spread as a dimmer for an incandescent lamp. Therefore, it is desirable that an illumination light source such as an LED can also be dimmed by the dimmer. As a power supply that is highly efficient and suitable for power saving and a reduction in size, a switching power-supply such as a DC-DC converter is known.
However, the dimmer is configured to operate while being connected in series to a filament of the incandescent lamp used as a load. If the switching power-supply is connected, it is likely that load impedance changes and the dimmer malfunctions.
If an optical output is reduced, in a switching operation, in a state in which an illumination light source is slightly lit, a switching regulator is operating or is stopped. Therefore, flickering tends to occurs. Even if current feedback is performed, it is difficult to detect a micro current during the slight lighting. The illumination light source tends to be unstably lit.